I was thinking of just taking the plastic buttplate off and screwing on a premade 1" or so thick recoil pad that follows the contour of the stock. Now that you mention it, I have one of the leather lace-on recoil pads with spacers made of delrin behind the buttplate under on my muzzleloader shotgun. Using one of those, I could use spacers and set the length of pull to whatever I wanted, I guess. The screw-on recoil pad seems a lot simpler, though, and would give me the 3/4" extra I need since I'd lose about 1/4" with taking the factory buttplate off and would gain 1" from the recoil pad.
I mostly just need to know what's legal by NRA rules that would let me lengthen the pull on my smallbore and pistol cartrige cowboy silhouette gun stocks. I'd be open to most anything that would fit the bill.